1. Field of the Invention
The present invention relates generally to variable displacement swash plate type compressors. More particularly, the invention relates to an improved mechanism for coupling a rotary journal in a variable displacement compressor with a drive plate which rotates integrally with a drive shaft.
2. Description of the Background Art
Variable displacement compressors have a wide variety of applications including the use as compressors for air conditioning, and refrigeration systems such as automotive air conditioners. Japanese Unexamined Utility Model Publication No. 62-183082 discloses a conventional variable displacement swash plate type compressor, which is illustrated in FIG. 6.
In this compressor, a rotary journal 103 is coupled, via a link pin 102, with a drive plate 101, securely mounted to a piston 106 that is connected to the swash plate 104 by a connecting rod 107. The rotational motion of the rotary journal 103 causes undulating movement of the swash plate 104, which in turn drives the connecting rods and pistons one after another in a linear reciprocating manner.
An arc-shaped elongated hole 109 is formed in a support arm 108, and protrudes from the drive plate 101. The elongated hole 109 serves as a guide for slidably holding the link pin 102. This arrangement keeps the top clearance of the piston 106 located at the top dead center position approximately constant, regardless of the inclination angle of the journal 103, and that of the swash plate 104.
When the dimensional accuracy of the arc-shaped, elongated hole 109 is low, however, the top clearance of the piston 106 cannot be maintained constant. Meanwhile, when the gap between the elongated hole 109 and the link pin 102 is great, noise is generated. It is difficult to further improve the dimensional accuracy of the elongated hole 109, with all the machining techniques currently available.
The discharge pressure of the compressor is generally higher than the internal pressure of the crank case. Therefore, when a certain piston is in its top dead position, the pressure of the gas in each compression chamber, that is exerted on the face of the piston head, will typically be higher than the pressure of the crank case gas acting on the rear side of the piston head. This creates a compressive stress which acts on the swash plate 104 at the point of action Mf.
In this design, the point of support Mk where the link pin 102 contacts the elongated hole 109 in the drive plate 101, will shift depending on the inclination angle of the swash plate 104. Especially in the type of compressor shown in FIG. 6, the line segment drawn between the point of action Mf and the point of support Mk is designed to be parallel with the drive shaft 100, when the inclination angle of the swash plate 104 is the greatest (when the discharge volume in the compressor is maximum). Accordingly, as the inclination angle of the swash plate 104 decreases with the change in the internal pressure of the crank case, the point of support Mk of the compressive stress shifts downward along the elongated hole 109. At the same time, the point of action Mf on the swash plate 104 receiving the compressive stress of the piston 106 located at the top position, shifts upward relative to the point of support Mk, so that the line segment drawn between the point of action Mf and the point of support Mk no longer maintains the parallel relationship with the drive shaft 100.
Thus, the compressive stress produces a moment that acts to further reduce the inclination angle of the swash plate 104. This moment makes it difficult to smoothly control the discharge volume of the compressor. In other words, although this moment promotes inclination of the swash plate 104 in the direction of decreasing discharge volume, it inhibits the inclination of the swash plate 104 in the direction of increasing the discharge volume. Therefore, such conventional compressor has two characteristics. The first is that compressor is sensitive to decrease in the discharge volume, and the second being that it is not sensitive to increase in the discharge volume. Such characteristics are not preferred in the variable displacement compressor.